1. Field of the Invention
The present invention relates to a yellow toner used for an image forming method such as an electrophotographic method, an electrostatic recording method, electrostatic printing method, or toner jet recording method; and an image forming apparatus using the yellow toner. The present invention is particularly suited for an image forming method using an oil-less fixing method.
2. Description of the Related Art
In full-color copying machines or printers, a desired color image is formed by using a cyan toner, a magenta toner, a yellow toner and a black toner, developing a latent image by superimposing the toners utilizing subtractive color mixing, transferring the respective toner images, which have been formed by the development, on a transfer material such as OHP sheet or plain paper to make the respective toner images should be finally superimposed, and then fixing the superimposed toner images onto the transfer material. Color toners therefore must have transparency so that the color of the upper toner layer does not disturb the color of the lower toner layer upon mixing. When the toner has poor transparency, the chromaticity of a projected image formed on an overhead projector (OHP) sheet changes, which prevents the formation of a desired color; or a color reproduction range becomes narrow because the color of the lower toner layer does not appear when the toners are superimposed. On-demand fixing system which hardly consumes energy during standby has recently been preferred from the ecological point of view. Compared with the ordinary roller fixing system, fixing tends to be performed under light pressure when on-demand fixing system is adopted. If a toner with poor transparency is used with the on-demand fixing system, the range of color reproduction further may be narrow.
In recent years, copying machines or printers which are more compact, lighter in weight, speedier and more reliable have been required eagerly from the space-saving and energy-saving viewpoints. Accordingly, their hardware has come to be formed of simpler elements and toners have been required to have higher performances. Without improvement in the performances of toners, it has been difficult to form an image with excellent quality. For example, with regards to power sources having an important role in the constitution of the hardware, use of one developing bias for the development of respective colors enables a reduction in the number of necessary power sources. In order to attain this, it is necessary to control the chargeability of respective color toners to be uniform.
In the investigation of colors or chargeability of toner, it must be considered that people have a high sensitivity to variations in the hue angle, particularly, of a yellow toner among various color toners, and are susceptible to a change in the chromaticity of a transmitted light. The chargeability of the yellow toner is much higher than that of a cyan toner or magenta toner. It was therefore necessary to make efforts to reduce the amount of a yellow colorant to be added to a toner. With a view to overcoming such a problem, use of a monoazo yellow pigment represented by the below-described formula is desired because it is excellent in the reflected color and coloring power. However, it has not been utilized fully for the toner because primary particles of the pigment tend to cause crystal growth upon drying or heating after its synthesis, which adversely affects the transparency of the toner. Problems to be solved are suppression of the pigment agglomeration and growth of primary particles and dispersing the pigment in the toner without increasing the particle size of the pigment.

A technology of improving dispersibility of pigment by synthesizing a pigment, mixing the resulting pigment, which is not pulverized but is in the hydrate form (paste pigment), with a resin under heating, and pelletizing the mixture into dry pellets; or by mixing a pigment powder with water and a resin under heating, and then pelletizing the mixture into dry pellets is proposed (for example, in Japanese Patent No. 2910945, and JP 06-148937 A, JP 6-161154, and JP 2002-129089 A).
Improvement in pigment dispersibility by incorporating an additive to a pigment (for example, in JP 7-128911 A) or by incorporating an additive upon kneading of a pigment and a resin (for example, in JP 7-28277 A and JP 9-258487 A) is also proposed.
According to the investigation by the present inventors, however, the above-described method such as use of a pigment in the paste form, addition of water upon kneading of a resin and a pigment, or use of an additive upon kneading of a resin and a pigment cannot suppress the growth of the primary particles of the pigment completely and there is still room for improvement in the transparency or color hue. In addition, the toner available by the above-described method does not have adequate charge response or a chargeability which can be controlled to match with that of the other color toners. Incorporation of a dissimilar raw material (additive) in order to heighten dispersibility improves dispersibility but changes the tint of the pigment itself or causes filming to a photosensitive drum.